The present invention relates to a method and apparatus for electroplating a substrate of electronic components, an IC wafer, a wafer of thin film magnetic heads and other substrates.
During electroplating, in order to prevent an abnormally plated surface to form, an anode electrode is arranged at the upper portion of a plating tank and an object to be plated is arranged at the lower portion of the tank so that the surface to be plated faces upwardly. If the surface to be plated is faced downwardly, hydrogen which generates from the cathode may contacts the surface to be plated causing abnormality of plated surface to occur.
Therefore, this kind of plating processes are carried out by a so called batch procedure consisting of removing plating solution from the tank at every completion of plating, supplying the plating solution again to fill the tank therewith after attaching a new object to be plated to the lower portion in the tank, and supplying plating current.
In order to allow uniform distribution of current density in a plating area so that the composition and thickness of a layer formed by plating (hereinafter called as a plated layer) can be uniformed, a ring-shaped cathode electrode which comes into face contact with a peripheral portion of the object to be plated is also provided (Japanese unexamined patent publication No.(1992)4-66698).
However, according to the above-mentioned conventional method, following problems occur when a batch procedure is carried out.
(1) An electrode film on the object to be plated, for flowing there through the plating current to the object, is etched while the plating solution is being supplied to the tank before plating.
(2) A plated layer formed on the object by plating is etched while the plating solution is being discharged after plating.
It is therefore an object of the present invention to provide an electroplating method and apparatus whereby, when an electroplating is carried out by a batch procedure, corrosion of an electrode film for passing plating current and of a plated layer on an object, which occurs in an off-state of plating electricity, can be prevented.
According to the present invention, a method of electroplating an object to be plated attached to a lower portion of a plating bath constituted by draining used plating solution and supplying new plating solution at every plating procedure is provided. The method has a step of supplying additional current so that the object to be plated always has cathode potential, while the object to be plated is being dipped in the plating bath and electroplating is not carried out.
According to the inventors of this application, the reason why an electrode film to conduct plating current to an object to be plated and a plated layer on the object are corroded during no current flowing, in case an electroplating of the object is carried out by a batch procedure have been found as follows. (1) Potential difference is generated between the object to be plated and cathode electrode while the object is anode potential, and (2) the plating solution itself is acidic (pH 2 to 3). Thus, according to the present invention, while the object to be plated is being dipped in a plating bath and no electroplating is carried out, additional current is supplied so that the object always has cathode potential in the plating bath. Consequently, there would occur no possibility of corrosion of the electrode film and the plated layer during no current supply. Furthermore, variation of thickness of a plated layer due to the corrosion can be drastically decreased, whereby film thickness controllability is largely improved.
Japanese unexamined patent publication No.(1988)63-111196 discloses a technique for preventing, in a horizontal continuous plating line for a steel sheet, melting of the upper plated layer by supplying current so that the electric potential of the steel sheet is kept negative with respect to that of the additional electrode. However, this technique aims to equalize the plating current for the upper plating surface with the plating current for the lower plating surface in the continuous plating process of steel sheets, and therefore there is no teach for preventing corrosion occurring during no current supply in the batch procedure as in the present invention.
While electroplating is being carried out, additional current may be supplied or may not be supplied.
It is preferred that additional current is direct current which is supplied from at least one additional electrode provided near the object to be plated in the plating bath in the direction of the object through the plating bath.
It is further preferred that the additional current is direct current with a current density of 0.01 to 0.1 A/dm2 (1 A/dm2=10 mA/cm2).
According to the present invention, an apparatus for electroplating an object to be plated is provided. The apparatus has a plating tank, a main anode electrode provided in the plating tank, a cathode electrode connected to the object to be plated and attached to a lower portion of the plating tank, a plating bath constituted by draining used plating solution and supplying new plating solution at every plating procedure, at least one additional anode electrode provided near the object to be plated in the plating tank, and an additional current source for supplying an additional current so that the object to be plated always has cathode potential, while the object to be plated is being dipped in the plating bath and electroplating is not carried out.
While the electroplating is being carried out, the additional current may be supplied or may not be supplied.
It is preferred that the additional current source is a source for supplying additional direct current with a current density of 0.01 to 0.1 A/dm2.
It is also preferred that the at least one additional current source includes a plurality of additional anode electrodes located around the cathode electrode while with a space there between. In this case, the additional anode electrodes may be located on the diagonal line of a bottom surface of the plating tank so as to sandwich the cathode electrode between them.
It is preferred that the at least one additional anode electrode is composed of platinum, nickel or titanium.
The cathode electrode is preferably composed of a metallic material with ionization tendency larger than that of a metallic material of an electrode film to conduct plating current to the object to be plated.
It is one of embodiments of the present invention that the object to be plated is a wafer for thin film magnetic heads.
Further, objects and advantages of the present invention will be apparent from description of the preferred embodiments of the invention as illustrated in the accompanying drawings.